Fix DISPLAY check + minor rewording of help texts

src/server/qgis_mapserver.cpp

@@ -6,6 +6,9 @@ The server listens to localhost:8000, the address and port can be changed with t
 environment variable QGIS_SERVER_ADDRESS and QGIS_SERVER_PORT or passing <address>:<port>
 on the command line.
 
+All requests and application messages are printed to the standard output,
+while QGIS server internal logging is printed to stderr.
+
                               -------------------
   begin                : Jan 17 2020
   copyright            : (C) 2020by Alessandro Pasotti
@@ -88,14 +91,14 @@ int main( int argc, char *argv[] )
   // minimal, minimalegl, offscreen, wayland-egl, wayland, xcb.
   // https://www.ics.com/blog/qt-tips-and-tricks-part-1
   // http://doc.qt.io/qt-5/qpa.html
-  const char *display = qgetenv( "DISPLAY" );
+  const QString display { qgetenv( "DISPLAY" ) };
   bool withDisplay = true;
-  if ( !display )
+  if ( display.isEmpty() )
   {
     withDisplay = false;
     qputenv( "QT_QPA_PLATFORM", "offscreen" );
-    QgsMessageLog::logMessage( "DISPLAY not set, running in offscreen mode, all printing capabilities will not be available.", "Server", Qgis::Info );
   }
+
   // since version 3.0 QgsServer now needs a qApp so initialize QgsApplication
   QgsApplication app( argc, argv, withDisplay, QString(), QStringLiteral( "QGIS Development Server" ) );
 
@@ -104,6 +107,12 @@ int main( int argc, char *argv[] )
   QCoreApplication::setApplicationName( "QGIS Development Server" );
   QCoreApplication::setApplicationVersion( "1.0" );
 
+  if ( ! withDisplay )
+  {
+    QgsMessageLog::logMessage( "DISPLAY environment variable is not set, running in offscreen mode, all printing capabilities will not be available.\n"
+                               "Consider installing an X server like 'xvfb' and export DISPLAY to the actual display value.", "Server", Qgis::Warning );
+  }
+
 #ifdef Q_OS_WIN
   // Initialize font database before fcgi_accept.
   // When using FCGI with IIS, environment variables (QT_QPA_FONTDIR in this case) are lost after fcgi_accept().
@@ -130,10 +139,10 @@ int main( int argc, char *argv[] )
   parser.addHelpOption();
   parser.addVersionOption();
   parser.addPositionalArgument( QStringLiteral( "addressAndPort" ),
-                                QObject::tr( "Listen to address and port (default: \"localhost:8000\")\n"
+                                QObject::tr( "Address and port (default: \"localhost:8000\")\n"
                                     "address and port can also be specified with the environment\n"
                                     "variables QGIS_SERVER_ADDRESS and QGIS_SERVER_PORT." ), QStringLiteral( "[address:port]" ) );
-  QCommandLineOption logLevelOption( "l", QObject::tr( "Sets log level (default: 0)\n"
+  QCommandLineOption logLevelOption( "l", QObject::tr( "Log level (default: 0)\n"
                                      "0: INFO\n"
                                      "1: WARNING\n"
                                      "2: CRITICAL" ), "logLevel", "0" );